Giant Faraday rotation in atomically thin semiconductors.

Faraday rotation is a fundamental effect in the magneto-optical response of solids, liquids and gases. Materials with a large Verdet constant find applications in optical modulators, sensors and non-reciprocal devices, such as optical isolators. Here, we demonstrate that the plane of polarization of light exhibits a giant Faraday rotation of several degrees around the A exciton transition in hBN-encapsulated monolayers of WSe 2 and MoSe 2 under moderate magnetic fields. This results in the highest known Verdet constant of -1.9 × 10 7  deg T -1  cm -1 for any material in the visible regime. Additionally, interlayer excitons in hBN-encapsulated bilayer MoS 2 exhibit a large Verdet constant (V IL ≈ +2 × 10 5  deg T -1  cm -2 ) of opposite sign compared to A excitons in monolayers. The giant Faraday rotation is due to the giant oscillator strength and high g-factor of the excitons in atomically thin semiconducting transition metal dichalcogenides. We deduce the complete in-plane complex dielectric tensor of hBN-encapsulated WSe 2 and MoSe 2 monolayers, which is vital for the prediction of Kerr, Faraday and magneto-circular dichroism spectra of 2D heterostructures. Our results pose a crucial advance in the potential usage of two-dimensional materials in ultrathin optical polarization devices.